A semiconductor module such as a power semiconductor module is equipped with a heat dissipation member (for example, a heat sink, a cooler) to release heat generated from a semiconductor element during its operation. In such a semiconductor module, a heat dissipation insulating substrate for transmitting the heat generated by the semiconductor element to the heat dissipation member is arranged between the semiconductor element and the heat dissipation member. The insulating substrate functions as a thermal conductor and also functions as an electrical insulator. Specifically, the insulating substrate includes a ceramic layer as an electrical insulating layer and a metal layer including a wiring layer (circuit layer) joined to one of surfaces of the ceramic layer (see, e.g., Patent Documents to 4). A semiconductor element is joined to the metal layer of the insulating substrate by soldering.
In the recent years, an Al layer made of Al or an Al alloy is used as a wiring layer because of the following reasons. That is, an Al layer is excellent in electrical characteristics and thermal characteristics, and is capable of reducing the production cost of the insulating substrate.
An Al layer, however, is poor in soldering joinability. Therefore, in cases where the wiring layer is an Al layer, in order to make it possible for a semiconductor element to be joined to the insulating substrate by soldering, for example, a Ni plated layer as a Ni layer is formed on the metal layer of the insulating substrate.
When the Ni layer, such as, e.g., a Ni plated layer, formed on the metal layer of the insulating substrate is about a few pm in thickness, there were following drawbacks. The heat applied during brazing of the insulating substrate and the heat dissipation member and the heat applied during soldering of the semiconductor element and the insulating substrate caused large irregularities on a surface of the Ni layer. This essentially prevented implementation of the semiconductor element to the insulating substrate. For this reason, it is preferable that the Ni layer is relatively thick. More specifically, it is especially preferable that the Ni layer is 15 μm or more in thickness.